1. Field of the Invention
This invention relates to an alumina sintered body made primarily of alumina and a manufacture thereof, and also to a spark plug using the alumina sintered body as an insulator thereof.
2. Technical Background
Alumina (Al2O3) is a material that has chemically and physically stable properties. An alumina sintered body made chiefly of alumina has been in wide use as an insulating material and the like in the fields of spark plugs used for combustion engines of automobile, engine parts, and IC boards because of its good insulating nature and withstand voltage characteristic.
It is known that a difficulty is involved in obtaining a dense sintered body, made primarily of alumina of high purity, at a relatively low temperature of not higher than 1650° C. To cope with this, the sintered body has been densified at such a relatively low temperature as mentioned above by addition of a sintering aid, such as silica (SiO2), magnesia (MgO), calcia (CaO) and/or the like, which is capable of forming a low melting liquid phase through eutectic reaction with alumina (see, for example, Japanese Laid-open Patent Application Nos. Sho 63-190753 and Hei 11-317279). The alumina sintered body obtained in this way is very stable thermally and chemically, high in mechanical strength and excellent in electric insulation. Thus, this sintered body has been widely put into practice as an electric insulator such as of spark plugs for combustion engines and the like.
On the other hand, internal combustion engines employed such as in automobiles have been advanced in respect of the power rating and miniaturization thereof. In association with an increasing valve occupying area in the combustion chamber, a demand for further miniaturization of spark plug is increasing. This entails a tendency toward thinning of an insulator interposed between a central electrode and a metal fitting serving as a ground electrode in the spark plug. Supercharging with a supercharger such as a turbo charger or the like is now in progress, with the tendency that a temperature within the combustion chamber increases. This in turn requires a high breakdown or withstand voltage, high heat resistance and high strengthening with respect to alumina sintered bodies used as an insulating material for spark plugs.
With existing alumina sintered bodies containing SiO2—MgO—CaO sintering aids illustrated as being conventional in Japanese Laid-open Patent Application No. Sho 63-190753, however, there exists or remains, as a low melting amorphous phase, the sintering aid in crystal grain boundaries of alumina. Such a low melting amorphous phase is so low in band gap that limitation is placed to achieve a high withstand voltage of the resulting alumina sintered body. In addition, the low melting amorphous phase is low in mechanical strength and places limitation on the high strengthening. Alumina has a high melting point (of about 2050° C.) and if the content of alumina in an alumina sintered body increases, a high withstand voltage and a high heat resistance can be expected. Nevertheless, a high content of alumina leads to poorer sinterability. This makes it difficult to provide a densely sintered product at a practical sintering temperature ranging from 1400° C. to 1600° C., thereby impeding to obtain a highly strengthened and high withstand voltage product.
Under these circumstance, Japanese Laid-open Patent Application No. Sho 63-190753 has realized an alumina ceramic having a porosity of not higher than 6% and a withstand voltage of 30 to 35 kV/mm and also a spark plug using this alumina ceramic as an insulator. In this application, there are used an alumina powder having an average particle size of not larger than 1 μm and a novel type of sintering aid made of at least one of yttria, magnesia, zirconia and lanthanum oxide employed in place of a conventional SiO2—MgO—CaO sintering aid thereby inhibiting abnormal grain growth of alumina by formation of a high melting grain boundary phase in the alumina grain boundaries.
In the Japanese Laid-open Patent Application No. Hei 11-317279, there has been proposed an alumina insulator having a high withstand voltage, which comprises from larger than 95 wt % to 99.7 wt % of alumina and from 0.3 to 5 wt % of a sintering aid. The sintering aid contains B component in an amount of from 0.01 to 0.25 wt %, converted as B2O3, so that alumina-based main phase grains having a grain size of not smaller than 20 μm are made at not less than 50% of the sectional area thereby making the number of pores having a size of not smaller than 10 μm at 100 or below per mm2.
In an example set out in this patent application, in spite of the fact that a penetrating insulation breakdown voltage in oil of a test piece shaped in a given size is a relatively high value of 38 kV and a spark plug is formed by use of a sintering aid containing a given ratio of boron and alumina, a withstand voltage test using an actual device reveals that spark penetration occurs in 40 to 50 hours after continuous operations at a discharge voltage of 35 kV.
This is considered for the following reason. When such a sintered body as set forth in the above-mentioned Japanese Laid-open Patent Application No. Hei 11-317279, in which alumina particles are grown into grains having a size as great as 20 μm or over in the course of sintering, is provided as a test piece of a simple form, the piece becomes high in green sheet density because of the relative ease in molding, thus resulting in a high sintered density to obtain a high withstand voltage. Nevertheless, if a configuration becomes relatively complicated as with the case of an actual spark plug, air existing in a molded body is not discharged along with the progress of sintering and thus, a number of pores are left in alumina particles having underwent a great degree of grain growth.
In fact, in examples of the Japanese Laid-open Patent Application No. Hei 11-317279 wherein the withstand voltage in an actual machine test becomes low, although it is not known exactly how many pores exist in an insulator of a spark plug, 91 or 92 pores having such a very great size of not smaller than 10 μm are observed per unit area of test pieces. Such pores existing inside the insulator may become a starting point for insulation breakdown when a high voltage is applied thereto.
Hence, it is considered that a difficulty is involved in completely removing pores inside the sintered body when using such a known procedure as set forth in the Japanese Laid-open Patent Application No. Hei 11-317279. If a test piece of a simple form exhibits a high withstand voltage, there is concern that withstand voltage may vary by the influence of the amount of pores left inside the sintered body although depending on the shape of product in case where an actual product is made by use of such a material that is difficult in removing the pores. This may impede the reliability as a spark plug.
In the sintering step of an alumina sintered body, where sintering is advanced while inhibiting grain growth as in the Japanese Laid-open Patent Application No. Sho 63-190753, open pores existing inbetween alumina particles move along with grain boundaries as the sintering proceeds and finally disappear. Nevertheless, although depending on the type and amount of sintering aid and the manner of addition, where a grain growth rate is higher than a moving rate of the pores under unsatisfactory conditions of suppressing the grain growth rate, there is concern that pores remain inside the grown alumina particles. Closed pores which have been once taken in the inside of the alumina particles do not disappear no matter how much it is heated subsequently and are left as defectives in the inside of the sintered body.
If a sintering aid is added in excess or if a sintering aid being added is locally distributed depending on the manner of addition, there is also concern that pores may remain because of insufficient sintering. Accordingly, in the alumina sintered body wherein a sintering aid that is able to inhibit grain growth of alumina is added as in the Japanese Laid-open Patent Application No. Sho 63-190753, there is some room for realizing a further high withstand voltage and high-strengthening by reducing defectives in number through improvement of a sintered density by optimizing the type of sintering aid to be used, mixing ratio and manner of mixing so as to improve densification of a sintered body while inhibiting the grain growth of alumina to promote elimination of the pores.